Time-of-flight mass separator having a flowing gas stream perpendicular to an ion drift field for increased resolution

ABSTRACT

Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions. Positive or negative ions of the trace gas are formed by ion-molecule reactions between the molecules of the trace gas and primary ions from a reactant gas. The ions are inserted in a stream of gas while subjected to an electric drift field and follow paths dependent upon their mass, the field strength, and gas flow-velocity. The field strength may be varied to produce successive outputs corresponding to different ion species at a predetermined region of the gas flow duct. Alternatively, simultaneous outputs for different ion species may be provided at different regions of the duct. The apparatus may be calibrated automatically by a feedback loop responsive to a known ion species.

United States Patent 1 1 3,596,088

[72] Inventors Martin J. Cohen [56] References Cited :3? '1"; m R F UNITED STATES PATENTS a n na; er

wmhmd Lake Worth, a f 2,950,387 8/l960 Brubaker t. 250 419 {Zl} Appl. No. 885,664 Primary Examiner-Archie R. Borchelt [22] Filed Dec. 17,1969 Assistant Examiner-C. E. Church [45] Patented July 27, 1971 Attorney-Raphael Semmes [73} Assignee Franklin GNO Corporation West Palm Beach, Fla.

Continuation-impart of application Ser. No.

9, v- 26, 1968- ABSTRACT: Apparatus and methods for sorting and detecting trace gases which undergo ion-molecule reactions. Positive or negative ions of the trace gas are formed by ionm m mm A ssizzxizzz zzzs zirazizni'gs'izz'is ffiz 2:12:2

v FLOWING GAS STREAM PERPENDICULAR To AN stream of gas while subjected to an electric drift field and fol- ION DRIFT FIELD FOR INCREASED RESOLUTION 26 Cw 6 D i low paths dependent upon their mass, the field strength, and gas flow-velocity. The field strength may be varied to produce [52] U.S. Cl 250/413 TF, cessive outputs corresponding to different ion species at a 250l4l.9 G, 250/419 811 predetermined region of the gas flow duct. Alternatively, [5|] lnt.Cl ..H01j 39/34, simultaneous outputs for difi'erent ion species may be pro- BOld 59/44 vidcd at different regions of the duct. Theapparatus may be [50] Field otSearch 250/419 calibrated automatically by a feedback loop responsive to a RECYCLE SYSTEM known ion speciesv PATf'NTEmuLz-mn SHEET 2 [IF 2 mvnmons MA'RTIN .J. COHEN M ED DAVID I. CARROLL ROGER F. WERNLUND IIIII ATTORNEY TIME-OF-FLIGII'I MASS SEPARATOR IIAVING A FLOWING GAS STREAM PERPENDICULAR TO AN ION DRIFT FIELD FOR INCREASED RESOLUTION REFERENCE TO COPENDING APPLICATION- This application is a continuation-in-part of Ser. No. 779,097, filed Nov. 26, 1968.

BACKGROUND OF INVENTION This invention relates to apparatus and methods of ion classification utilizing gas flow as a measurement parameter. More particularly, the invention is concerned with the detection of trace vapors which undergo ion-molecule reactions and with the separating and measuring of molecular quantities of trace substances in a gaseous sample.

It has heretofore been proposed to measure ion mobility by ionizing molecules of a stream of gas and subjecting the ions to an electric drift field, which may be transverse to the gas flow direction or opposed to the gas flow. Reference is made, for example, to Physical Review, Feb. 1929, Vol. 33, p. 21 7 et seq. More recently, the standard electron capture detectors have utilized air flow in a rough way to separate electrons and ion components of current for purposes of detection of the reduction in current when electron carriers attach to become ions. However, simple and practical instruments employing gas flow as a measurement parameter have not heretofore been available for highsensitivity, high-resolution ion detection and classification.

The aforesaid copending application discloses improved apparatus and methods employing gas flow as a quantitative parameter in the detection of trace gases which are capable of being electrically charged.

BRIEF DESCRIPTION OF THE INVENTION The present invention is concerned with improvements upon and modifications of the apparatus and methods disclosed in the aforesaid copending application, and it is accordingly a principal object of the invention to provide such improved apparatus and methods.

Briefly stated, preferred embodiments of the apparatus and methods of the invention are concerned with "Plasma Chromatography systems involving the formation of either positive or negative ions by reactions between the molecules of the trace substances and primary ions. The secondary ions may then be separated, detected, and measured. Separation is accomplished by utilizing the difierence in velocity of ions of different mass in an electric field applied to a stream of gas into which the ions are injected. In a preferred form of the invention, primary or reactant ions are formed by electron attachment, for example, to the molecules of a reactant gas. A drift field causes the primary ions to migrate through a reaction chamber, during which the primary ions react with molecules of a trace gas to be detected to form secondary trace gas or product ions. The secondary ions and any remaining primary ions pass through an ion-transmissive aperture into a stream of gas and drift transversely of the stream. Ions having a predetermined mobility follow a path which leads to an ion detector. An ion drift velocity spectrum is provided by varying the drift voltage to produce successive outputs for different ion species or by providing multiple detector components for concurrently detecting plural ion species. For purposes of calibration, the ion drift voltage may be controlled automatically by a feedback arrangement which maintains a constant output of a known ion.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein:

FIG. 1 is a plan view of the basic apparatus in accordance with the invention as disclosed in the aforesaid copending application;

FIG. 2 is a somewhat diagrammatic side elevation view, partly in section, illustrating the basic apparatus of the invention;

FIG. 3 is a transverse sectional view taken along line 3-3 of FIG. 2;

FIGS. 4, 5 and 6 are diagrammatic views illustrating modifications of the basic apparatus in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION The copending application of Martin J. Cohen, David 1. Carroll, Roger F. Wernlund, and Wallace D. Kilpatrick, Ser. No. 777,964, filed Oct. 23, 1968 and entitled Apparatus and Methods for Separating, Concentrating, Detecting and Measuring Trace Gases," discloses Plasma Chromatography systems involving the formation of primary ions and the reaction of such primary ions with molecules of trace substances to form secondary ions, which may be concentrated, separated, detected, and measured by virtue of the difference in velocity or mobility of the ions in an electric field. As set forth in the said copending application, the broad principle of ionmolecule reactions is well documented in the literature, but the utilization of this principle in high-sensitivity systems for detecting and measuring trace substances is novel. Primary ions may be produced by subjecting the molecules of a suitable host gas, such as air, to ionizing radiation, such as beta rays from a tritium source, corona from a multipoint or wire array, electrons produced by photoemission from a cathode, etc. The primary ions are then subjected to an electric drift field, causing them to migrate in a predetermined direction through a reaction space into which the sample or trace gas is introduced. The resultant collisions between the primary ions and the molecules of the sample gas produce secondary ions of the sample gas in much greater numbers than can be produced by mere electron attachment, for example, to the trace gas molecules. The secondary ions are also subjected to the electric drift field and may be sorted in accordance with their velocity or mobility. The specific systems of the said copending application Ser. No. 777,964 employ ion shutter grids or gates for segregating the ion species in accordance with their drift time. In the present invention, the secondary ionsare injected into a stream of gas and are sorted in accordance with their velocity under the combined influence of the drift field and the gaseous flow.

Referring to the drawings, apparatus in accordance with the preferred form of the invention comprises a duct or tunnel 10 of rectangular cross section, having a first pair of parallel sides 12 and 14, which may be termed the top and bottom, and a second pair of parallel sides 16 and 18, which may merely be termed sides. The duct has an outlet section 20 and an outlet section 22. The inlet section has a transition portion 24 which contracts transversely in two dimensions from the upstream to the downstream end of the transition portion. The main section of the duct between the inlet and outlet sections is of uniform rectangular cross section. The outlet section 22 may contract transversely in a horizontal plane, as indicated by the transition portion 26 in FIG. 1, and may expand in a vertical plane as shown in FIG. 2. The outlet section may include an isolation sleeve 28 leading to a housing 30 containing a blower which may exhaust to the atmosphere or which may be part of a recycling system indicated diagrammatically at 32 in FIG. 2.

The top and bottom walls 12 and 14 of the main section of the duct are preferably of conductive material, while the remaining sidewalls l6 and 18 are preferably constituted by alternating strips of conductive material 34 and insulation 36. From FIG. I it can be observed that the structure comprising the alternate strips 34 and 36 is external to the main gas flow passage through the duct and hence does not produce turbulence in the duct, the sidewalls of which are essentially smooth and continuous. The inlet section 20 contains a series of parallel screens or grids 38 arranged transversely of the gas flow and serving to reduce turbulence. The use of screens 38 together with the transition portion 24 (which preferably has a cross section contraction ratio of about 4), the smooth sidewalls of a relatively long main duct section, and the use of relatively low gas flow velocity reduce turbulence and eddies to a minimum in order to preserve uniform gas flow velocity profile through the main section of the duct.

Adjacent to the top of the duct and external of the main gas flow passage is an ion-molecule reaction chamber 40 into which a sample and host gas may be introduced by means of an inlet conduit 42. Chamber 40 communicates with the interior of the duct through a small slit or ion-transmissive window 44 formed in a conductive portion of the top wall 12 between a pair of spaced insulator portions 48. Chamber 40 contains a principal electrode 50 (a cathode if negative ions are produced or an anode if positive ions are produced) which may have an opening 52 through which gas may enter the chamber space. An ionizing means, such as a tritium strip on electrode 50, is provided in the region of the electrode. Arcuate focusing grids are provided at 54 and 56. Gas may exhaust from the chamber 40 through a separate duct 58 running external of the top wall 12 into the outlet 22 or exiting to the atmosphere. The relative pressures in the chamber 40 and the duct 10 may be maintained by standard flow control techniques so as to minimize the leakage of neutral molecules through slit 44. If desired, the slit may be covered by a thin ion-permeable membrane.

The bottom wall 14 is provided with a slit 60 which leads to a dead air space 62 in which is located an ion sensor, which may comprise electrode 64 of an electrometer-type ion detector. The main section of the duct 10 may be divided longitudinally by a central plate or diaphragm 66 parallel to the top and bottom walls 12 and 14 and having a transverse opening 68 for the passage of ions. The diaphragm assists in preserving uniformity of the electric drift field which will now be considered.

The electric drift field may be produced and maintained uniform by applying appropriate electric potentials to electrode 50, to wall 12, bottom wall 14, electrometer electrode 64, and the conductive strips 34, which constitute electric guard slats. Any suitable DC supply provided with a voltage divider, such as a resistor chain, may be employed. When negative ions are produced, the bottom plate 14 of the duct may be at ground potential, the top plate 9,000 v. negative relative to ground, the cathode 50 9,l v. negative relative to ground, the electrometer electrode 64, 100 v. positive relative to ground, and the guard slats 34 at voltages increasingly negative with respect to ground (within the 0 to 9,000 v. range) from the bottom wall 14 to the top wall 12. For use with positive ions, the polarity is reversed. If the top and bottom walls proper are electrified, rather than separate electrode sections, the ends of these walls must obviously be insulated from each other.

In the operation of the apparatus of the invention, a host gas carrying an ionizable gaseous trace sample, for example, air carrying SO or Ethion, enters the reaction chamber 40 through inlet 42. The host gas, or components thereof such as oxygen or nitrogen, is ionized adjacent to electrode 50 by ionizing means. As pointed out in the said copending application Ser. No. 777,964, ions of the plentiful host gas molecules form preferentially. The primary ions then migrate toward the aperture 44 under the influence of the drift field, the voltage gradient being relatively low in the reaction chamber. In traversing the reaction chamber 40, the primary ions undergo ion-molecule reactions with the molecules of the trace gas, producing secondary ions of the trace gas. The mean free path of the ions in the reaction space is preferably very much smaller than the dimensions of the reaction space, a condition which prevails if the chamber 40 is maintained at about atmospheric pressure, for example. The resultant ions are focused upon aperture 44 by grids 54 and 56 and pass through the aperture into the stream of gas drawn through duct by the blower. The ions drift downstream and toward the bottom wall 14 under the combined influences of the gas flow and the drift field, the path followed by the ions being a function of their velocity. Some of the ions will follow paths ending upon the bottom wall 14, where the ions will be neutralized. Predetermincd species of ions, depending upon ion mass, gas flow velocity, and drift field parameters, will pass through aperture 60 and impinge upon electrode 64, producing an output current which is measured by a suitable detection circuit, such as an integrator. By virtue of the utilization of the ionmolecule reaction principle to produce large numbers of trace ions and by utilizing low-velocity uniform gas flow profile, high sensitivity and high resolution are attainable. Detectable output currents for particular ion species are produced at significantly lower trace concentration than with comparable apparatus known heretofore.

The gas employed in the duct or tunnel 10 may be cleaned, adjusted in temperature and pressure, and flow-controlled by the recycle system 32. Furthermore, the tunnel gas may be different from the host gas supplied to the reaction chamber and may be inert with respect to ion-molecule reactions involving the primary and secondary ions of interest, thereby to serve as a quenching medium for terminating ion-molecule reactions and for restricting the same to the chamber 40. The tunnel gas may be recycled separately from the host and sample gases, which may have their own pump or pressure source. The use of an ion-reaction region within chamber 40 which is quite small permits rapid sample change from a sample source with a small total quantity of available trace, such as a gas chromatograph.

As indicated above, in accordance with the invention, ion species are separated under the combined influence of an electric drift field and a gaseous stream. The ion velocity in the field direction is equal to the ion mobility multiplied by the field strength. The ion velocity in the gas flow direction is equal to the flow velocity. The resultant ion velocity is the vector sum of these two components. The velocity of the ion in the gas flow at atmospheric pressure is independent of the ion mass. The velocity of the ion in the electric field is dependent upon the mobility, which is a function of both the ion and carrier gas masses. The mobility at very low concentrations in air is given for normal gases by the Langevin relationship:

where B is an empirical constant, m, the mass of the carrier gas molecules, and m, the mass of ions. For air, m, is 28.55 and B is approximately 1.96. The ion velocity vector due to the electric field is thus:

VE /1+28.55/mi where E is the electric field. Since the electric field velocity vector is dependent upon the ion mass, the resulting vector sum will be also. The transit time of an ion between the top and bottom wall electrodes of the duct is independent of gas flow velocity and is equal to the electrode spacing divided by V The transit time rnultiplied by the gas flow velocity v will give the distance travelled by the ion perpendicular to the electric field. THus, if the ion receiver slot is positioned downstream of the ion injection slot by a distance appropriate to the desired ion mass, it will receive only ions of the selected mass. lons of lower mass will not travel far enough to reach the receiver slot, while ions of higher mass will travel too far to reach the slot.

In a typical apparatus of the type illustrated in the drawings, the inlet section may be 23 inches wide by 13 inches high, 7 inches long for the uniform cross dimension portion, and 8 inches long for the transition section. The length of the main duct section may be 40 inches long, the internal width 16 inches and the height 5 inches. Slit 44 may be located at about the longitudinal center of the top wall of the main duct section. The center of slit 68 may be displaced downstream 2.5 inches from the center of slit 44, while the center of slit 60 may be displaced downstream 2.5 inches from the center of slit 68. The width of slit 44 may be 0.040 inch. The thickness of the sides of the duct constituted by the alternate strips of conductive and nonconductive material may be 2 inches. Typically, the ion source may be a seven square centimeter strip of tritium, the air stream velocity 1,000 centimeters per second, the electric drift field between the top and bottom walls of the duct 760 v. per centimeter, the working pressure 760 torr. Sensitivity to a selected trace material may be one part in The ion velocity filter approach of the present invention offers the potential for much higher sensitivities, by virtue of the continuous ion source, than is possible with pulsed ion sources. Another advantage is that all ions which do not enter the collector slot are intercepted by the adjacent anode (or cathode) plate and eliminated from the signal.

Among the many applications of the invention is the analysis of stack gas for S0 and 80,, content. In stack gases, most sulfur already exists in the form of SO, and S0 The remainder, if any, can be converted to $0 by a simple catalytic oxidation flow cell. A small volume of the stack gas is obtained from across the stack by a rack-type inhaler to obtain a representative sample. The sample is filtered of gross particulate and passed through a heated catalytic oxidizer to convert any remaining sulfur aerosol, hydrogen sulfide and carbon-sulfur compounds to sulfur dioxide. The gases may then be diluted by ten times or more with fresh air and inserted into the reaction chamber.

The apparatus of FIGS. 1-3 may be employed in a system which provides an ion drift velocity spectrum. This may be accomplished as shown in HO. 4, for example, by employing a drift voltage source which is made variable by means of a voltage scanner connected to the various electrodes indicated. If a variable voltage is applied across the fiow Plasma Chromatography system of FIGS. l-3, the position of the ions can be moved along the bottom surface of the structure, the particular ion species which enters the bottom slit 60 varying with the drift voltage. Thus, by scanning the drift voltage, an entire ion drift velocity spectrum may be produced in the output as a function of time.

FlG. 5 illustrates an alternative method of obtaining an ion drift velocity spectrum. Here, a series of output slits 60 is provided in the lower wall 14' of the duct, corresponding output electrodes 64' being provided, one for each slit. If it is desired to divide the spectrum into 100 ion drift time units, for example', then 100 slits would be needed. Although 100 separate output electrometers or the like could be provided, a simpler system may employ a device (such as the Waveform Eductor of Princeton Applied Research Corp.) in which the current from each output electrode charges a corresponding capacitor, the capacitors being sampled in turn by a single electrometer, which is scanned through the sequence.

FIG. 6 illustrates a method of calibrating the apparatus. In a simple system in which the amount of a particular ion, such as positive oxygen or nitrogen ions, is known, it is merely necessary to adjust the drift voltage to a value at which the output from a particular slit (which senses the known ion) is kept constant. This simple feedback arrangement, which can compensate for changes in gas flow rates, for example, may be refined by the use of a dual-slit, where the standard" ion peak has to balance equally between the two slits. Thus, as shown in FIG. 6, the lower wall 14" of the duct is provided with a dual-slit (two closely spaced slits) 60" and corresponding electrodes 64' which feed a comparator. The output of the comparator may be a null or zero voltage when the peak is equally distributed between the slits 60", but otherwise may produce a correction voltage of appropriate polarity and magnitude for application to a drift voltage control, which varies the drift voltage to maintain the peak centered on the dual-slit. The usual output (which may include multiple slits as shown in FIG. 5) is indicated at 60 and 64.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims.

The invention 1 claim is:

l. A method of analyzing different-mobility ions which comprises inserting the ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a predetermined region, subjecting said ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the ions drift to locations dependent upon their mobility, the field strength, and the gas flow velocity, selectively sensing the ions which reach a predetermined location transversely of said stream in the direction of said field and downstream of said predetermined region, producing an electrical output in response to the sensed ions, and varying the drift field during the said sensing and while producing said electrical output.

2. A method in accordance with claim 1, wherein the drift field is varied so as to cause ions of different mobility to reach said location at different times.

3. A method in accordance with claim I, wherein the drift field is varied in response to the ions sensed at said predetermined location.

4. A method in accordance with claim 3, wherein the drift field is varied so as to maintain the number of ions sensed at said predetermined location substantially constant.

5. A method in accordance with claim 1, wherein a particular species of ions is sensed at said predetermined location and a further location closely adjacent thereto, and wherein the drift field is varied in response to the difference in the number of ions sensed at said locations.

6. A method in accordance with claim 1, wherein at least some of said ions are product ions formed by reacting molecules of a sample with reactant ions.

7. A method in accordance with claim 6, wherein ion formation occurs at a region external to said gas stream but coupled thereto.

8. A method in accordance with claim 7, wherein the product ions are formed in a first gaseous medium and said gas stream comprises a second gaseous medium.

9. A method in accordance with claim 8, wherein the second gaseous medium is inert with respect to the ionmolecule reactions.

10. A method in accordance with claim 6, wherein the ionmolecule reactions occur in a chamber and the gas stream flows in a cut substantially larger than the chamber.

11. A method in accordance with claim 1, wherein the subjecting of said ions to said drift field comprises applying to said ions a steady unidirectional drift field during the other steps recited.

12. A method of analyzing different-mobility ions which comprises inserting the ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a predetermined region, subjecting said ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the ions drift to locations dependent upon their mobility, the field strength, and the gas flow velocity, selectively sensing the ions which reach a plurality of predetermined locations transversely of said stream in the direction of said field and downstream of said predetermined region, and producing electrical outputs in response to the sensed ions at said locations, respectively.

13. A method in accordance with claim 12, wherein at least some of said ions are product ions formed by reacting molecules of a sample with reactant ions.

14. A method in accordance with claim 12, wherein the subjecting of said ions to said drift field comprises applying to said ions a steady unidirectional drift field during the other steps recited.

15. A method of calibrating apparatus in which ions of different mobility are inserted at a predetermined region in a substantially nonturbulent continuous gas stream of predetermined flow rate, subjected to an electric drift field transverse to the gas stream, and analyzed in accordance with their mobility in the stream and field, which comprises selectively sensing a known ion species adjacent to a different predetermined region of the stream spaced transversely from the firstmentioned region in the direction of said field and located downstream of said first-mentioned region, and controlling the drift field in response to the sensing of said known ion species.

16. Apparatus for analyzing ions of different mobility, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined flow rate, means for inserting said ions into said duct at a first region, means for applying an electric drift field to said ions inserted into said duct, said field being transverse to said flow, means adjacent to another region of said duct spaced transversely from the first region in the direction of said field and located downstream of the first region for selectively sensing ions which reach said other region by following paths dependent upon their mobility in the gas flow in said duct and the drift field, and means for varying the strength of said drift field during the said sensing.

17. Apparatus in accordance with claim 16, further comprising means for producing at least some of said ions as product ions by ion-molecule reactions in a chamber adjacent to one side of said duct, said chamber having means for receiving a gaseous sample and having means associated therewith for producing reactant ions which react with molecules of said sample to form said product ions.

7 18. Apparatus in accordance with claim 17, wherein said chamber is located exteriorly of said duct at one side thereof and said sensing means is located at the opposite side thereof.

19. Apparatus in accordance with claim 18, wherein said duct is of rectangular cross section and in which each of the remaining sides of the duct comprises alternating strips of electrically conductive and electrically insulating material parallel to the first-mentioned sides and located outside of the flow path through said duct.

20. Apparatus in accordance with claim 18, wherein said duct is provided with a longitudinally extending central plate parallel to said sides of the duct and having an ion-transmissive aperture therein for passage of ions from said chamber to said sensing means.

21. Apparatus in accordance with claim 17, wherein said chamber has means for applying a drift field to ions therein to urge them toward said duct.

22. Apparatus in accordance with claim 17, wherein said reactant ion-producing means comprises a continuous ionizing source.

23. Apparatus in accordance with claim 16, further comprising means for controlling the drift field in response to said sensing.

24. Apparatus in accordance with claim 23, wherein said sensing means has means for producing a pair of electrical outputs from closely adjacent locations and for comparing said outputs, and said controlling means is responsive to the comparison.

25. Apparatus for analyzing ions of different mobility, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined flow rate, means for inserting said ions into said duct at a first region, means for applying an electric drift field to said ions inserted into said duct, said field being transverse to said flow, and a plurality of means adjacent to further regions of said duct spaced transversely from the first region in the direction of said field and located successively downstream of the first region for selectively sensing ions which reach said further regions by following paths dependent upon their mobility in the gas flow in said duct and the drift field.

26. Apparatus in accordance with claim 25, further comprising means for producing at least some of said ions as product ions by ion-molecule reactions in a chamber adjacent to one side of said duct, said chamber having means for receiving a gaseous sample and having means associated therewith for producing reactant ions which react with molecules of said sample to form said product ions. 

1. A method of analyzing different-mobility ions which comprises inserting the ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a predetermined region, subjecting said ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the ions drift to locations dependent upon their mobility, the field strength, and the gas flow velocity, selectively sensing the ions which reach a predetermined location transversely of said stream in the direction of said field and downstream of said predetermined region, producing an electrical output in response to the sensed ions, and varying the drift field during the said sensing and while producing said electrical output.
 2. A method in accordance with claim 1, wherein the drift field is varied so as to cause ions of different mobility to reach said location at different times.
 3. A method in accordance with claim 1, wherein the drift field is varied in response to the ions sensed at said predetermined location.
 4. A method in accordance with claim 3, wherein the drift field is varied so as to maintain the number of ions sensed at said predetermined location substantially constant.
 5. A method in accordance with claim 1, wherein a particular species of ions is sensed at said predetermined location and a further location closely adjacent thereto, and wherein the drift field is varied in response to the difference in the number of ions sensed at said locations.
 6. A method in accordance with clAim 1, wherein at least some of said ions are product ions formed by reacting molecules of a sample with reactant ions.
 7. A method in accordance with claim 6, wherein ion formation occurs at a region external to said gas stream but coupled thereto.
 8. A method in accordance with claim 7, wherein the product ions are formed in a first gaseous medium and said gas stream comprises a second gaseous medium.
 9. A method in accordance with claim 8, wherein the second gaseous medium is inert with respect to the ion-molecule reactions.
 10. A method in accordance with claim 6, wherein the ion-molecule reactions occur in a chamber and the gas stream flows in a cut substantially larger than the chamber.
 11. A method in accordance with claim 1, wherein the subjecting of said ions to said drift field comprises applying to said ions a steady unidirectional drift field during the other steps recited.
 12. A method of analyzing different-mobility ions which comprises inserting the ions into a substantially nonturbulent continuous gas stream of predetermined flow rate at a predetermined region, subjecting said ions to an electric drift field transverse to said gas stream while they are in said stream, whereby the ions drift to locations dependent upon their mobility, the field strength, and the gas flow velocity, selectively sensing the ions which reach a plurality of predetermined locations transversely of said stream in the direction of said field and downstream of said predetermined region, and producing electrical outputs in response to the sensed ions at said locations, respectively.
 13. A method in accordance with claim 12, wherein at least some of said ions are product ions formed by reacting molecules of a sample with reactant ions.
 14. A method in accordance with claim 12, wherein the subjecting of said ions to said drift field comprises applying to said ions a steady unidirectional drift field during the other steps recited.
 15. A method of calibrating apparatus in which ions of different mobility are inserted at a predetermined region in a substantially nonturbulent continuous gas stream of predetermined flow rate, subjected to an electric drift field transverse to the gas stream, and analyzed in accordance with their mobility in the stream and field, which comprises selectively sensing a known ion species adjacent to a different predetermined region of the stream spaced transversely from the first-mentioned region in the direction of said field and located downstream of said first-mentioned region, and controlling the drift field in response to the sensing of said known ion species.
 16. Apparatus for analyzing ions of different mobility, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined flow rate, means for inserting said ions into said duct at a first region, means for applying an electric drift field to said ions inserted into said duct, said field being transverse to said flow, means adjacent to another region of said duct spaced transversely from the first region in the direction of said field and located downstream of the first region for selectively sensing ions which reach said other region by following paths dependent upon their mobility in the gas flow in said duct and the drift field, and means for varying the strength of said drift field during the said sensing.
 17. Apparatus in accordance with claim 16, further comprising means for producing at least some of said ions as product ions by ion-molecule reactions in a chamber adjacent to one side of said duct, said chamber having means for receiving a gaseous sample and having means associated therewith for producing reactant ions which react with molecules of said sample to form said product ions.
 18. Apparatus in accordance with claim 17, wherein said chamber is located exteriorly of said duct at one side thereof and said sensing means is located at the opposite side thereof.
 19. Apparatus in accordance with claim 18, wherein said duct is of rectangular cross section and in which each of the remaining sides of the duct comprises alternating strips of electrically conductive and electrically insulating material parallel to the first-mentioned sides and located outside of the flow path through said duct.
 20. Apparatus in accordance with claim 18, wherein said duct is provided with a longitudinally extending central plate parallel to said sides of the duct and having an ion-transmissive aperture therein for passage of ions from said chamber to said sensing means.
 21. Apparatus in accordance with claim 17, wherein said chamber has means for applying a drift field to ions therein to urge them toward said duct.
 22. Apparatus in accordance with claim 17, wherein said reactant ion-producing means comprises a continuous ionizing source.
 23. Apparatus in accordance with claim 16, further comprising means for controlling the drift field in response to said sensing.
 24. Apparatus in accordance with claim 23, wherein said sensing means has means for producing a pair of electrical outputs from closely adjacent locations and for comparing said outputs, and said controlling means is responsive to the comparison.
 25. Apparatus for analyzing ions of different mobility, which comprises a duct having means for producing a substantially nonturbulent continuous gas flow therethrough at a predetermined flow rate, means for inserting said ions into said duct at a first region, means for applying an electric drift field to said ions inserted into said duct, said field being transverse to said flow, and a plurality of means adjacent to further regions of said duct spaced transversely from the first region in the direction of said field and located successively downstream of the first region for selectively sensing ions which reach said further regions by following paths dependent upon their mobility in the gas flow in said duct and the drift field.
 26. Apparatus in accordance with claim 25, further comprising means for producing at least some of said ions as product ions by ion-molecule reactions in a chamber adjacent to one side of said duct, said chamber having means for receiving a gaseous sample and having means associated therewith for producing reactant ions which react with molecules of said sample to form said product ions. 